1. Field of the Invention
The present invention relates to a semiconductor device.
2. Description of the Background Art
A current-driven bipolar transistor is known as an exemplary conventional semiconductor device. In a conventional bipolar transistor, a diffusion current is generated due to difference between the impurity concentrations of an emitter layer and a base layer, and an electron current (emitter current) flows from the emitter layer to the base layer, while a hole current (base current) flows from the base layer to the emitter layer. The gain of the current of the bipolar transistor is generally represented by a value obtained by dividing a collector current by the base current. The collector current and the emitter current are substantially identical with each other, and hence it may be considered that the gain of the current of the bipolar transistor is a value obtained by dividing the emitter current by the base current. In the conventional bipolar transistor, difference is provided between the impurity concentrations of the emitter layer and the base layer, and the current of the bipolar transistor is amplified due to generation of the difference between the emitter current and the base current. In recent years, the resistance of the base layer is required to be reduced in order to improve high speed response (high frequency characteristic) of the bipolar transistor, and therefore a larger quantity of the impurity is implanted into the base layer. However, implantation of a large quantity of the impurity increases the base current, and hence the gain of the current of the bipolar transistor is disadvantageously reduced.
In relation to this, there has generally been proposed a structure capable of reducing the gain of the current of the bipolar transistor while increasing the impurity concentration of the base layer of the bipolar transistor and reducing the resistance of the base layer. In this conventional semiconductor device, a base layer consisting of SiGe is employed. The band gap of Ge is smaller than that of silicon, and hence the band gap of SiGe is an intermediate value between silicon and Ge. Therefore, the band gap of the base layer is smaller than that of an emitter layer or a collector layer consisting of Si. Thus, energy difference on a side closer to a valence band of the boundary between the emitter layer and the base layer is increased, and hence movement of holes (hole current) from the base layer to the emitter layer is reduced to some extent as compared with a case of employing a base layer consisting of silicon. Consequently, the movement of holes is reduced to some extent even when the impurity concentration of the base layer is increased in order to reduce the resistance of the base layer, and hence the gain of the current of the bipolar transistor can be reduced.
In the aforementioned conventional semiconductor device, however, it is disadvantageously difficult to sufficiently suppress the movement of holes between the emitter layer and the base layer.